1. Field of the Invention
This invention relates to apparatus for measuring, observing, or carrying out operations on work pieces composed of various elements and materials such as various semiconductor elements and semiconductor materials, superconducting materials, or other metallic or organic materials, etc., at temperatures as low as a boiling point of liquified gas, while cooling the work piece so as to keep the work piece at a low temperature. 2. Description of the Prior Art
Recently, high sensitivity fluxmeters referred to as SQUID (superconducting quantum interferometer) microscopes having a spatial resolution in the order of micrometers have been implemented, with measurements of various elements and materials using a Squid microscope becoming commonplace. SQUIDs use superconductors, and therefore have to be cooled to a low temperature (from a few K to 77K) lower than the temperature of liquid nitrogen. It is also often necessary to keep aworkpiece at a low temperature. In addition to SQUID, there are also cases where a work piece is maintained at a low temperature during observation by a tunnel microscope or an atomic force microscope.
FIG. 2 is a schematic view showing an example of a related cooling apparatus for cooling a sensor side to a low temperature. A tri-axial scanning stage 20, cooling head 30, storage tank 40, sensor 50, and work piece 60 etc. are located within a vacuum chamber 10, and a vacuum pump 70, liquid helium container 80, and vacuum insulation piping 90 are located outside of the vacuum chamber 10.
The vacuum chamber 10 is made of stainless steel, with a vacuum being maintained therein in order to provide thermal insulation with the outside.
The tri-axial scanning stage 20 is installed with the work piece 60, and is used to control the relative positions of the sensor 50 and the work piece 60.
The cooling head 30 is made of pure copper and maintains a state of thermal contact with the sensor 50.
The storage tank 40 holds a refrigerant for cooling the cooling head 30. In order to use liquid helium as a coolant, a connection is made with the liquid helium container 80 using the vacuum insulation piping 90 and transferring is carried out in order to save liquid helium in the storage tank 40. In order to reduce infiltrating heat, a heat insulating coolant tank 41 is located around the storage tank 40 and holds liquid nitrogen.
A SQUID having a detection coil with a diameter in the order of 10 mm is employed as the sensor 50. Niobium operating at the melting point of liquid helium is employed as the superconducting material from which the SQUID is made.
By actuating the vacuum pump 70, coolant stored in the storage tank 40 is conveyed through piping 31 to the cooling head 30. After the cooling head 30 is cooled, the coolant is discharged from piping 32 to the outside through the vacuum pump 70.
In a procedure for measuring distribution of the magnetic field of the work piece 60, liquid nitrogen is saved to the storage tank 40 and the heat insulating coolant tank 41, and the periphery of the storage tank 40 is cooled down to the melting temperature of liquid nitrogen. Liquid nitrogen that has entered the storage tank 40 is then removed, the storage tank 40 and the liquid helium container 80 are connected by vacuum insulation piping 90, and liquid helium constituting the coolant is transferred to the storage tank 40. After this, the vacuum pump 70 is actuated, liquid helium flows through to the cooling head 30, and the cooling head 30 is cooled down to in the region of the melting temperature of helium. The sensor 50 is then actuated, the relative positions of the sensor 50 and the work piece 60 are controlled using the tri-axial scanning stage 20, and the measuring can be carried out by recording a signal from the sensor 50.
With the related cooling apparatus where it is necessary to cool the sensor or work piece to a low temperature, it is necessary to transfer liquid helium to a storage tank equipped with a vacuum chamber and keep it there. It is also necessary to remove liquefied gas directly prior to the transfer of liquid helium after introducing liquefied gas such as liquid nitrogen to the storage tank once prior to transfer and pre-cooling the storage tank down to the melting temperature of liquefied gas. It is also necessary to fill up the heat insulating coolant tank 41 located around the periphery of the storage tank with liquefied gas such as liquid nitrogen. Cooling of the sensor or work piece down to a low temperature is therefore both troublesome and time consuming, and it is also necessary to prepare a coolant such as liquid nitrogen, etc. Further, because it is necessary to temporarily hold liquid helium in the storage tank, particularly in cases where the time taken to cool the sensor or work piece down to a low temperature is short, the amount of liquid helium consumed in order to cool the storage tank in comparison to the amount of liquid helium consumed in order to cool the cooling head cannot be ignored, and the loss of liquid helium is substantial as a result. Moreover, the vacuum chamber is large because a liquid helium storage tank is provided at the vacuum chamber, which also makes the footprint of the cooling apparatus substantial.
Rather than providing a storage tank for liquid helium within the vacuum chamber, a liquid helium introduction port is provided taking into consideration heat insulation. A liquid helium container and the port are then connected by vacuum heat insulating piping so that liquid helium is supplied directly from the container.